commit dcffee067290c03f89c5afcacab1ba543ee28e7c (tree)
parent ba4af7aa8a33d19bbf7b0225a2d1407d5a756c06
Author: mihael <hi@mihaelm.com>
Date: Wed, 18 Mar 2026 20:32:18 +0100
`libzigc/math`: Implement `frexp`
The changes were tested by running:
```
$ ./build/stage3/bin/zig build -p stage4 -Denable-llvm -Dno-lib
$ stage4/bin/zig build test-libc -Dlibc-test-path=<LIBC-TEST-PATH> -Dtest-filter=frexp -fqemu -fwasmtime --summary line
Build Summary: 737/737 steps succeeded
```
The tests were passing even when it was a straightforward calling of Zig
std library, but I wanted the `x is NaN` special case to match the
behaviour described in `libc` manpages, and for it to be consistent
with how infinities as arguments are handled in Zig.
Diffstat:
4 files changed, 20 insertions(+), 25 deletions(-)
diff --git a/lib/c/math.zig b/lib/c/math.zig
@@ -60,6 +60,7 @@ comptime {
symbol(&exp10, "exp10");
symbol(&exp10f, "exp10f");
symbol(&fdim, "fdim");
+ symbol(&frexp, "frexp");
symbol(&hypot, "hypot");
symbol(&modf, "modf");
symbol(&pow, "pow");
@@ -161,6 +162,25 @@ fn fdim(x: f64, y: f64) callconv(.c) f64 {
return 0;
}
+fn frexp(x: f64, e: *c_int) callconv(.c) f64 {
+ // libc expects `*e` to be unspecified in this case; an unspecified C value
+ // should be a valid value of the relevant type, yet Zig's std
+ // implementation sets it to `undefined` -- which can even be nonsense
+ // according to the type (int). Therefore, we're setting it to a valid
+ // int value in Zig -- a zero.
+ //
+ // This mirrors the handling of infinities, where libc also expects
+ // unspecified for the value `*e` and Zig std sets it to a zero.
+ if (math.isNan(x)) {
+ e.* = 0;
+ return x;
+ }
+
+ const r = math.frexp(x);
+ e.* = r.exponent;
+ return r.significand;
+}
+
fn hypot(x: f64, y: f64) callconv(.c) f64 {
return math.hypot(x, y);
}
diff --git a/lib/libc/musl/src/math/frexp.c b/lib/libc/musl/src/math/frexp.c
@@ -1,23 +0,0 @@
-#include <math.h>
-#include <stdint.h>
-
-double frexp(double x, int *e)
-{
- union { double d; uint64_t i; } y = { x };
- int ee = y.i>>52 & 0x7ff;
-
- if (!ee) {
- if (x) {
- x = frexp(x*0x1p64, e);
- *e -= 64;
- } else *e = 0;
- return x;
- } else if (ee == 0x7ff) {
- return x;
- }
-
- *e = ee - 0x3fe;
- y.i &= 0x800fffffffffffffull;
- y.i |= 0x3fe0000000000000ull;
- return y.d;
-}
diff --git a/src/libs/musl.zig b/src/libs/musl.zig
@@ -839,7 +839,6 @@ const src_files = [_][]const u8{
"musl/src/math/__fpclassify.c",
"musl/src/math/__fpclassifyf.c",
"musl/src/math/__fpclassifyl.c",
- "musl/src/math/frexp.c",
"musl/src/math/frexpf.c",
"musl/src/math/frexpl.c",
"musl/src/math/i386/acosl.s",
diff --git a/src/libs/wasi_libc.zig b/src/libs/wasi_libc.zig
@@ -701,7 +701,6 @@ const libc_top_half_src_files = [_][]const u8{
"musl/src/math/finitef.c",
"musl/src/math/fma.c",
"musl/src/math/fmaf.c",
- "musl/src/math/frexp.c",
"musl/src/math/frexpf.c",
"musl/src/math/frexpl.c",
"musl/src/math/ilogb.c",
